In recent years, optical fibers of new configurations called holey fibers or photonic crystal fibers have been becoming a focus of attention (see Non Patent Literature 1, for example).
A typical configuration of a holey fiber is illustrated in FIG. 13. As illustrated in FIG. 13, a holey fiber 103 is comprised of a core 131 and a cladding 132 formed around the core 131, wherein the cladding 132 has a plurality of air holes 133 that extends longitudinally along the axis of the core 131.
The air hole 133 in the cladding 132 of the holey fiber 103 allows ingress of water thereinto or invites occurrence of condensation therein due to temperature variation if the air hole 133 is open to the outside at the fiber end. These disadvantages will possibly cause lowering mechanical strength or variation in optical properties.
In splicing optical fibers using such as a mechanical splice and an MT connector, such splicing practice fills the gap between the end faces of optical fibers to be spliced together (splicing end faces) with a refractive index matching liquid to reduce reflection and loss at the splice. Where the optical fiber to be spliced is a holey fiber like the one illustrated in FIG. 13 however, the refractive index matching liquid will flow into inside of the air holes from the end face of the splice. This behavior invites an anxiety about causing a large reflection or splice loss at the splicing end face. Further, an optical connector for mono-fiber splicing, which needs no refractive index matching liquid, has problems in that the polishing operation applied on the splicing end face causes polishing agent and polishing chips to enter the air hole inviting degradation of optical properties and appear at a later stage on the splicing interface disturbing light propagation.
JP 2004-4320 A discloses a method that deals with this problem. The method is to seal the end of the air hole of a holey fiber with a sealing material having a refractive index lower than that of the core of the fiber.
JP 2002-323625 A discloses other different methods to seal the air holes of the holey fiber. The methods include: collapsing the air holes by heating around the cladding at the part a little distant from the splicing face; closing air holes by heating the splicing face with a fusion splicer (a splicing device for splicing optical fibers by heat-fusion with atmospheric discharge); blocking the air holes by filling the holes with curable resin such as ultraviolet ray curable resin and heat curable resin; and sealing the air hole by covering the end thereof with metallic thin film.
The method of closing the air holes by collapsing the air holes by heating around the cladding as disclosed in JP 2002-323625 A is advantageous compared to the method of blocking the air holes by filling the holes with a curable resin in that there is no anxiety about aged deterioration and that polishing the end face is easy. This method however has a problem in that collapsed portion needs cutting operation to make the portion to be the splicing face because the collapse-fused portion is local.
As a method of sealing the air holes by fusing the holey fiber itself, it would be feasible to splice an optical fiber of ordinary structure to the holey fiber. This method however tends to cause axial or angular deviation or bulge at the splicing point. The occurrence of these deviations or bulge at the splicing point brings problems in workability or reliability of the splice in that the holey fiber will encounter inexpediency in inserting into a ferrule and that such inexpedient situation will easily result in damage on the holey fiber surface. Further, this method has another disadvantage in that the splice as a whole will involve unavoidable additional splice loss caused by splicing an optical fiber of ordinary structure to the holey fiber.
Therefore, it is the most suitable and practical method to block using curable resin for sealing the air holes of the holey fiber. In particular, the ultraviolet ray curable resin is considered to be the optimal material from viewpoints:
(1) Viscosity before being cured is low, which makes filling into air holes easy;
(2) Irradiation of ultraviolet light cures the resin in a short time; and
(3) The resin is a field-proven material as an adhesive for optical use.    {Non Patent Literature 1} HASEGAWA Takemi: “Recent advances in photonic crystal fibers and holey fibers” Monthly OPTRONICS No. 7 (2001) pp. 203-208; The Optronics Co., Ltd.